In 1988, the United States produced a larger volume of polymers and plastics than the entire annual production volume of steel, aluminum, and copper combined. Polymers have become the most common material in our daily lives and are critical items of commerce, industry, and technology. Plastics are a subset of all polymers and have the following characteristics: 1) high molecular weight molecules, 2) the molecules are composed of smaller units, called the repeat unit, which is chemically bound to other repeat units to make up the molecule, 3) the solid is often above its glass transition temperature when at room temperature, and 4) the solid is extrudible at temperatures above its application temperature and is thus a thermoplastic.
Methods of making polymers are very important for industrial chemical synthesis processes and industrial materials. However, most previous and current synthesis processes make homopolymers of ethene, petroleum-based monomers. These homopolymers contain only one repeat unit in the molecule. As these oil-based monomers become more expensive and more scarce, the polymers we use will have to be made from other natural products.
The three natural polymers available to form commercial plastics are cellulose, lignin, and starch. Methods to create sidechains on cellulose and starch by free radical polymerization are known. But, these methods fail to attach sidechains to lignin because the reactions which produce reactive sites on anhydroglucose chains (cellulose or starch) are not powerful enough to do the same thing on lignin.
In U.S. Pat. No. 4,687,828, entitled WATER SOLUBLE GRAFT COPOLYMERS OF LIGNIN-(2-PROPENAMIDE)-(SODIUM 2,2-DIMETHYL-3-IMINO-4-OXOHEX-5-ENE-1-SULFONATE), METHODS OF MAKING THE SAME AND USES THEREFOR, Meister and Patil described methods for preparing a specific copolymer of lignin and claimed a product grafted lignin copolymer so produced. The method of the 828 patent and the lignin copolymer produced thereby differ from the instant invention. The product of the instant invention is new and has been produced by a new, unique method.
Some common products which contain lignin are mechanical pulp, thermomechanical pulp, wood, or wood fiber. However, these lignin containing materials have hydrophilic surfaces which are incompatible with hydrophobic materials such as plastics. Hydrophilic means that a material is "water seeking or water preferring". The chemical structure of a hydrophilic material produces a lower Gibbs free energy (lower total energy) when the material interacts with, imbibes, or wets with water. Hydrophilic materials: 1) have a contact angle of less than 90.degree. with water (wet), 2) may dissolve in water, 3) may imbibe water, and 4) may be hydroscopic (prone to extract water from humid air). Hydrophobic means water repelled. Hydrophobic materials will increase total system energy when placed in contact with water. Thus, these materials are minimally soluble, if not insoluble, in water; have contact angles above 90.degree. with water (non-wetting); and tend to repel water.
To reinforce a hydrophobic plastic with a fiber like thermomechanical pulp and wood, the fiber surface must be made compatible with the plastic. Otherwise, plastic and pulp won't bind to one another and there will be no reinforcing effect from adding the pulp to the plastic. The use of lignin as a basic building block to form plastics will resolve an important disposal problem, and accordingly have a significant positive impact on the environment, as will be understood from the following discussion concerning the production and disposal of lignin during wood/cellulose processing.
To obtain cellulose from natural materials containing lignocellulose, the material is subjected to a chemical treatment which solubilizes the lignin to a degree which will allow the cellulose to be separated in the form of fibers. The dissolved lignin constitutes between about 25 to 45% of the lignocellulose, the amount depending on the extraction process used, the extent of the solubilization, and the sources of the lignocellulose undergoing separation. The lignocellulose used in most processes is usually wood trees and the trees used to supply the lignin are usually classified as soft and hard woods.
Lignin has such a limited commercial utility that its disposal has become a source of serious ecological and economic problems. In the past, most of the lignin solutions have been sewered or pumped directly into rivers and streams, destroying the ecological balance of the environment. Small amounts of lignin are used as drilling muds or are calcined to yield adsorbent, activated carbons. Much of it is concentrated by evaporation to a lower water content and sprayed into a furnace to burn the lignin, and from the ashes thus produced, a partial amount of the inorganic chemicals used in the process is recovered. The amount of lignin used in these applications represents a very small fraction of the millions of tons of lignin generated yearly as a by-product of the pulp and paper industry. Stringent anti-pollution regulations have obligated this industry to commit about one-half billion dollars a year for pollution control, the cost of which led to severe economic problems which resulted on the assumption of these expenditures by the industry. The burning of the lignin for its fuel value and for the recovery of inorganic minerals is insufficient to recover the cost of the pollution control.
Eventually, the total cost of producing cellulose products from lignocellulose materials will be borne by the ultimate consumer in the form of higher prices for the products, unless substantial credit can be obtained by upgrading or modifying the lignin through a high volume utilization. Lignin can be recovered from pulping operations in the form of brown amorphous powder, if it is dried to eliminate substantial quantities of water or extraction solvent.
The lignin is obtained as a by-product of any of the processes used industrially to obtain cellulose from lignocellulose compositions. In the sulfite processes, sulfonate moieties are attached to coniferyl units in the lignin and act as solvating groups to produce a water-soluble lignin; the water-insoluble form can be produced from these soluble lignins by acidification or by other chemical treatments. The alkaline process for preparing cellulose is more efficient than the sulfite process and produces higher yields of cellulose fiber. In the alkaline process, liquors containing either sodium hydroxide or a mixture of sodium hydroxide and sodium sulfide are used to produce "alkali lignin" as a lignin salt which is soluble in the pulping liquor, from which it can be recovered conveniently by acid precipitation; the lignin isolated depends upon the specific conditions which the lignin is obtained. Thus, if it is precipitated at a pH in the range of 9.5 to 10.0, a lignin salt is obtained, but if it is precipitated at a low pH, below 7.0, and washed thoroughly with water, a free lignin is obtained. By adjustment of the pH, fractions of various molecular weights can also be obtained and isolated. Most of the paper pulp, of the order of 90% or more, is produced in this country by the Kraft process, with liquors containing sodium sulfide. Unfortunately, sodium sulfide simultaneously produces dimethyl sulfide and methyl mercaptan. To reduce or overcome this odor problem, some pulps are manufactured by the peroxide process, which is based on hydrogen peroxide adjusted to specific pH values. Minor amounts of lignin are obtained at the present time, from the peroxide processes, but the volume is expected to increase.
Lignin [8068-00-6] is derived from woody plants. In fact, after cellulose, it is the principal constituent of the woody structure of higher plants. Lignin, which makes up about 25% of the weight of dry wood, acts as a cementing agent to bind the matrix of cellulose fibers together into a rigid woody structure. See Biochemistry by A. L. Lehninger (Worth Publishers, 1970).
Moreover, lignin sources are abundant. Although the wood and bark waste from the lumber industry and wastes from agricultural operations could provide extremely large quantities of lignin, perhaps the most accessible, albeit smaller, source is the pulp and paper industry. For example, in 1978, it has been estimated that the U.S. chemical-pulp industry produced 1.55.times.10.sup.7 tons of alkali lignin and 1.6.times.10.sup.6 tons of lignosulfonic acids. See Encyclopedia of Chemical Technology, vol. 14 (Kirk-Othmer, 1981).
In general, the molecular structure of the repeating lignin units and the appropriate numbering thereof is as follows: ##STR5##
It appears that, regardless of origin, lignin [8068-00-6] is a complex, oxyphenylpropene polymer. In the natural state, lignin is a highly branched and partially cross-linked polymer. However, there appears to be some structural variation in branching depending upon whether the lignin is derived from coniferous or deciduous species or from bark, cambium, sapwood or heartwood. During recovery, the lignin is chemically altered and is available in relatively pure form as a derivative having a molecular weight of about 1,000 to 150,000. Of the lignins which may be used according to the present invention, there may be mentioned alkali lignins, HCl lignins, acid lignins, Klassen lignins, solvent-extracted lignins, steam-explosion lignins, milled wood lignins (MWL) and 1,4-dioxane lignins, for example, with each lignin named according to the method of recovery used to obtain it. Methods for recovering lignin are the alkali process, the sulfite process, ball milling, enzymatic release, hydrochloric acid digestion, and organic solvent extraction. Alkali lignins are produced by the kraft and soda methods for wood pulping. They have low sulfur content (&lt;1.6 weight percent), sulfur contamination present as thioether linkages, and are nonionic polymers of low (2,000 to 15,000) molecular weight. Alkali lignins are tan, brown or black powders. When free of metal cations, such as sodium or potassium, alkali lignins are water-insoluble materials and are commonly called "free acid" or "acid free" lignin. When containing metal cations, such as sodium or potassium, the alkali lignins are slightly water soluble materials which increase in water solubility as the pH increases from 7 toward 14 and become completely soluble in 5 weight percent aqueous sodium hydroxide solutions. Approximately 20 million tons of kraft lignin are produced in the United States each year.
The sulfite process for separating lignin from plant biomass produces a class of lignin derivatives called lignosulfonates. Lignosulfonates contain approximately 6.5 weight percent sulfur present as ionic sulfonate groups. These materials have molecular weights up to 150,000 and are very water-soluble. Lignosulfonates are used in resource recovery as cement grouting agents, sacrificial agents in EOR, and thinning agents in drilling muds. The material is therefore directly utilized in energy recovery.
Milled wood lignin (MWL) is produced by grinding wood in a rotary or vibratory ball mill. Lignin can be extracted from the resulting powder using solvents such as methylbenzene or 1,4-dioxacyclohexane. Milling only releases 60 weight percent or less of the lignin in wood, disrupts the morphology of lignin in wood, and may cause the formation of some functional groups on the produced lignin. Despite these limitations, milling appears to be an effective way of recovering lignin from plants with only slight alteration. Enzymes which hydrolyze polysaccharides can be used to digest plant fiber and release lignin. After digestion, the lignin is solubilized in ethanol. Extensive analytical studies support the idea that enzymatically produced lignin has undergone no major modification in removal from plant material. Milling and enzyme release are not commercial methods to recover lignin at present, but the commercialization of ethanol from biomass processes may make enzyme lignin available in large quantities.
Acid hydrolysis of the polysaccharide portion of wood will release lignin but also causes major condensation reactions in the product. These reactions can be minimized by using 41 wt. percent hydrochloric acid in place of other mineral acids but some condensation reactions still occur. This is not an effective method by which to obtain unaltered lignin. On the other hand, lignin can be solvent extracted from wood at temperature of 175.degree. C. using solvent mixtures such as 50/50 by volume water/1,4-dioxacyclohexane. Changes in lignin under these conditions appear to be minor. All of these lignins can be used as raw materials for graft copolymerization and none is automatically preferred over the others. Choice of lignin to be used in the reaction is made on the basis of availability, cost, and the properties desired in the final copolymer. When the product to be made is to have high molecular weight, a high molecular weight lignin is usually chosen as a starting material. If the product to be made is to have low molecular weight, a low molecular weight lignin is usually chosen as a starting material. If the product to be made is to be a highly ionic, conducting copolymer; a highly ionic, conducting lignin such as a lignosulfonate is usually chosen as a starting material. If the product to be made is to be a non-ionic thermoplastic, a non-ionic lignin is usually chosen as a starting material. Rules such as these are general indications of how to choose a lignin for use in the grafting reaction and are the technical underpinning by which the examples, to be shown later, are designed.
The aromatic ring of a lignin repeat unit is often alkoxy substituted, as shown in the structure above, and the propene group often has a hydroxyl group attached in place of one hydrogen. Alkyl groups appear on some of the aromatic groups of the polymer and sulfur may be chemically bound to parts of the polymer, though few, if any, sulfonate groups occur.
Bonding between repeat units in alkali lignin is complex and involves carbon-carbon bonds between aromatic and/or alkyl carbons as well as ether bonds between aromatic and/or alkyl carbons. Labile hydrogens exist in the material and may be replaced by metal cations, such as sodium, potassium, calcium, or ammonium ions, to form alkali lignin salts. Alkali lignins are readily identified by method of production and are a familiar class of compounds to those versed in the paper making art. Steam explosion lignins are prepared from steam explosion pulp by any of the lignin extraction methods described previously. Steam explosion pulp is made by heating wood to a temperature at which water would boil if exposed to conditions which prevail in the next stage of the production process. The wood is then thrust into this "next stage" and the spontaneous formation of steam bursts the wood and produces a pulp.
Lignins are polymeric substances composed of substituted aromatics found in plant and vegetable tissue associated with cellulose and other plant constituents. In the pulp and paper industry, lignin-containing materials such as wood, straw, corn stalks, bagasse, and other vegetable and plant tissues are processed to recover the cellulose or pulp. The residual pulping liquors containing the lignin as by-products are thus one of the main sources of lignins. While there is some variation in the chemical structure of lignin, depending upon the plant from which lignin is obtained, place where the plant is grown, and also upon the method used in recovery or isolation of the lignin from the plant tissue, the basic structure and properties of the lignins are similar, all containing the alkane-substituted, phenolic groups. Thus, lignins obtained by any method or from any source may be used in this reaction as long as the lignin is in a form: 1) soluble in an aqueous alkaline medium or other solvent or 2) suspendable in emulsion, suspension, or neat polymerization reaction.
Since the lignins separated from the plant may be chemically altered somewhat from that found in the plant, the term "lignins", as used herein, means lignin of or from the lignin containing materials mentioned above. Lignin products may be products obtained upon separation from the cellulose or recovered from the plant. In the sulfite pulping process, the lignocellulosic material is digested with a bi-sulfite or sulfite resulting in the sulfonation of the lignins. In other methods of the recovery or separation of the lignins from the plant, the lignins may not be sulfonated but may be chemically altered somewhat in some other manner. For example, in residual pulping liquors obtained in the sulfate and other alkaline pulping processes, the lignins are present as alkaline metal salts dissolved in the alkaline aqueous liquor. "Hydrolysis lignin" obtained from the hydrolysis of lignocellulosic materials in the manufacture of sugar is likewise altered somewhat from that found in the plant. Also the lignin products such as a residual pulping liquor may be subjected to various treatments such as, for example, acid, alkaline or heat treatments or reacted with other chemicals which may further alter somewhat the lignin constituents. The lignins remain operative for this process as long as the treatment is not so severe as to destroy the basic polymeric structure or substantially decrease the phenolic hydroxyl content of the lignin.
The residual pulping liquors, or the lignin-containing product obtained in the separation or recovery of lignins from the plant, will generally contain lignins of various molecular weights varying from less than 1,000 to over 100,000. These liquors also may contain other constituents besides lignins. For example, in the sulfite pulping process, the spent sulfite liquor contains lignosulfonates which may be present as salts of cations, such as magnesium, calcium, ammonium, sodium and other cations which may have been present during the sulfonation of the lignin. The spent sulfite liquor generally contains only about 40 to 60 weight percent of an oven dried basis of lignosulfonates with the remainder being carbohydrates and other organic and inorganic constituents dissolved in the liquor. Lignin products obtained by other pulping processes may likewise contain other materials such as carbohydrates, degradation products of carbohydrates, and resinous materials which are separated from the lignocellulosic materials with the lignin. Alkaline treatment of the lignins has a tendency to increase the phenolic hydroxyl content and also to enhance the flocculating properties of the final product under alkaline conditions.
Treatments of lignin by high-energy, ionizing radiation such as x-rays or gamma rays can connect sidechains to lignin but these reactions create crosslinked solids that lack thermoplastic properties and can not be dissolved. As will be shown below, a method has now been developed which allows ethene monomers to be attached to lignin and lignin thermoplastics to be made. This invention provides a broad spectrum of soluble, extrudeable copolymers and methods for making said copolymers, which improves upon prior methods. See Tables 1 and 2 below.
The invention provides a series of methods for synthesizing a lignin graft copolymer or modifying the surface of a wood fiber or pulp and to provide a spectrum of reagents to use in the processes for preparing the lignin graft copolymer, grafted wood, grafted wood fiber, or grafted pulp and to provide a method of boosting or enhancing polymer molecular weights during polymerization reactions.
In accordance with the present invention, to the lignin macromolecule, possibly to the aromatic ring of the oxyphenylpropene moiety, is grafted 1) repeating units of 1-(pendant group)ethylene: ##STR6## 2) a combination of randomly occurring repeating units of 1-(pendant group)ethylene with 1-(alternate pendant group)ethylene: ##STR7## 3) a combination of randomly occurring repeating units of 1-methyl-1-(pendant group)ethylene with 1-(alternate pendant group)ethylene: ##STR8## or 4) a combination of randomly occurring repeating units of 1-methyl-1-(pendant group)ethylene with 1-methyl-1-(alternate pendant group)ethylene: ##STR9## In these formulas, m and n are integers varying from 1 to 300,000. R.sup.1 l and R.sup.2 are organic functional groups which do not interfere with free radical polymerization, and the structures presented represent typical polymer or random-sequence, copolymer sidechains attached to lignin by the process of this invention.
The method of preparing a copolymer of lignin basically comprises:
1) providing an oxygen free environment; PA1 2) forming a reaction mixture of:
a) a lignin source PA2 b) a redox initiator PA2 c) a halide salt PA2 d) at least one monomer selected from the group of CH.sub.2 :CHR.sub.1 and CH.sub.2 :CHR.sub.2, as described hereinabove.
The objectives of the present invention include: 1) provide new copolymers containing lignin as the backbone component with a side chain or chains formed from a vinyl monomer; and 2) methods for synthesizing lignin copolymers having vinyl monomer side chain or chains.
Accordingly, it is an object of the present invention to provide a series of methods for synthesizing a lignin graft copolymer or modifying the surface of a wood fiber or pulp. It is also an object of the present invention to provide a spectrum of reagents to use in the processes for preparing the lignin graft copolymer, grafted wood, grafted wood fiber, or grafted pulp. Further, it is also an object of the present invention to provide a method of boosting or enhancing polymer molecular weights during polymerization reactions.